The present invention relates to the improvement of the performance characteristics of a thin-film transistor. It relates to, in particular, a method for reducing contact resistance made on the boundary surface between an electrode and a semiconductor.
With transformation into an information society, there is noted the development of, for example, thin and light electronic paper as a substitute for paper and ID tags that permit momentary discrimination of commodities from one another. At present, in these devices, a thin-film transistor (TFT) obtained by using amorphous silicon (a-Si) or polycrystalline silicon (p-Si) as a semiconductor is used as a switching element. However, the production of TFT using such a silicon semiconductor entails a great cost because it requires a plasma chemical vapor deposition (CVD) apparatus, a sputtering apparatus and the like, which are expensive. Moreover, the TFT is produced through many processes such as vacuum process, photolithography, processing, etc., resulting in a low throughput.
Therefore, there is noted organic TFT obtained by using as a semiconductor layer an organic material which can be formed by printing and makes it possible to provide a product at low cost. JP-A-2000-307172 discloses a bottom contact structure as a typical structure of organic TFT. It is known that organic semiconductor materials are generally inferior to inorganic semiconductors in chemical resistance and heat resistance. Electrodes and an insulating film are formed by a high-temperature process and wet etching or a coating process. Therefore, in the case of organic TFT comprising a mixture of an organic semiconductor, a metal for electrodes, and another organic material for an insulating film or the like, an organic semiconductor film is liable to be deteriorated during a process of forming individual layers. For these reasons, the bottom contact structure formed by forming an organic semiconductor layer after forming a gate electrode, a gate insulator, a source electrode and a drain electrode on an insulating substrate is suitable for organic TFT.
IEEE TRANSACTION ON ELECTRON DEVICE, VOL. 48, NO. 6, p. 1060 reports the following: in organic TFT having the bottom contact structure, a gold source electrode and a gold drain electrode are modified with a self-aligning monomolecular film (SAM) of a thiol to change the surface energy of the gold and increase the particle size of pentacene crystals grown on the electrodes, whereby the mobility in the TFT is increased.
In the case of the bottom contact structure described above, when the semiconductor layer is formed of an organic material capable of undergoing polycrystal growth, such as pentacene, the size of semiconductor crystals on the source and drain electrodes is less than one-tenth that on the insulating film. As a result, there is the following problem: a large number of grain boundaries are present in a channel region near the boundary between each of the source and drain electrodes and the semiconductor, so that the contact resistance on the boundary surface between each of the source and drain electrodes and the semiconductor is increased. Carriers present between the source electrode and the drain electrode and on the boundary surface between the organic semiconductor layer and the gate insulator on the gate electrode assume an accumulated state or a depleted state, depending on the polarity and magnitude of a voltage applied to the gate electrode, so that they modulate the electric current between the source electrode and the drain electrode. The current path of about 10 nm in thickness described above is a channel.
When the gold surface is densely modified with SAM of a thiol, the work function of the gold is decreased by about 0.5 eV, so that the Schottky barrier on the boundary surface between gold and pentacene is undesirably increased. Moreover, when the gold electrode is modified with molecules with a large molecular length of octadecanethiol or the like, the distance between gold and pentacene is increased, so that the injection of carriers into the semiconductor from the electrode is undesirably disturbed.
An object of the present invention is to provide a high-performance semiconductor device obtained by reducing the contact resistance on the boundary surface between each of source and drain electrodes and semiconductor of organic TFT having the bottom contact structure, and using the organic TFT as a switching element.